Alex V. Levin, MD, MHSc, FRCSC
Department of Ophthalmology
The Hospital for Sick Children
University of Toronto

Dilating drops (also called “mydriatics”) are used to enlarge the pupil for eye examinations to allow the eye doctors to see into the eye in detail. The eye doctor can then see the optic nerve and retina as well as the whole lens. Dilating drops work either by temporarily paralyzing the muscle that makes the pupil smaller (iris sphincter muscle) or by stimulating the iris dilator muscle. Patients with lighter eye color (blue, hazel) are more sensitive and dilate faster than patients with darker eye color (brown).
Mydriatics can be used for the following reasons.

Some of the mydriatics also relax the focusing muscles of the lens of the eye. This allows the eye doctor to get a most accurate measurement of the correct glasses prescription. Note that this does not apply to children who have had their natural lens removed as a result of cataract surgery.

Keeping the pupil big is important after surgery to prevent scar tissue formation (synechia). This is especially true for children with iritis. However, mydriatics are not always used after eye surgery as in some cases pupil dilation is not desired.

Making the pupil big before cataract surgery makes it easier for the surgeon to remove the lens of the eye (the cataract is in the lens).

If a cataract is small, enlarging the pupil can allow the patient to see ‘around the cataract’ and perhaps avoid surgery. Likewise, in patients with ‘out of position’ lenses (ectopia lentis) this can help make the vision clearer by allowing the patient to ‘look around’ the edge of the lens.

After glaucoma surgery, mydriatics help the eye to stay well formed and recover when the eye pressure gets too low.

Some forms of glaucoma are due to the iris and pupil coming too far forward. Dilating drops help to restore the normal anatomy of the eye and treat/prevent this kind of glaucoma (closed angle glaucoma). This is sometimes the case in children with total retinal detachment following retinopathy of prematurity (ROP), PHPV, or retinoblastoma (an eye tumour in children).

As dilating drops can cause temporary blurry vision, in children they can be used to blur the good eye to force the brain to use the bad eye as a treatment for amblyopia (‘lazy eye’).

All dilating drops make the pupils large and may make eyes more sensitive to bright lights. The duration of this effect depends on the particular eye drop that is used. It is advisable to wear sunglasses to minimize glare from bright lights if this symptom is bothersome.

All eye medications may result in unwanted side effects in the rest of the body as a result of their absorption into the blood from blood vessels of the eye. Therefore, only apply one drop at the specified times instructed by your doctor. This is especially important in infants. As with all medicines, keep out of the reach of children. Atropine in particular is extremely toxic and can kill if swallowed.

Side effects of mydriatics include:

Whitening of the eyelids and skin around the eyes. This is completely harmless and will resolve spontaneously. It is due to the constriction of small skin blood vessels.

Atropine can cause redness/flushing of the face, a warm sensation of the skin to touch, irritability or sweating. Although not dangerous, it does mean that a lower dose should be considered. Stop the medication and inform your eye doctor.

All mydriatics sting for a few seconds after they are put in the eye

Other side effects are very rare. They include high blood pressure, confusion, hallucinations, and seizures. But this rarely occurs beyond infancy and usually only in premature babies. Eye doctors avoid these effects by using weaker concentrations in premature babies. Itchy rashes can also rarely occur.

It is important to wash your hands thoroughly after giving these drops to prevent inadvertent dilation of your own pupils by touching your eyes!

There are two main groups of dilating drops [not all drug company names listed]:

Parasympathetic antagonists (parasympatholytics): act by paralyzing the iris sphincter muscle. This category of medicines will both make the pupil larger and paralyze the muscle involved in focusing of the lens (accommodation). As a result, they will cause the eye to be blurry especially for up close (reading, near play).

Tropicamide: (drug company names include Tropicamide, Mydriacyl, Tropicacyl) Drops available as 0.5% (used in premature infants) and 1%. Duration of action: up to 6 hours.

Homatropine: (Isopto Homatropine) Drops available as 2% and 5%. The lower strength is usually preferred in children. Duration of action: 2-3 days.

Atropine: (Atropisol, Isopto Atropine) Drops available as 0.5% or 1% (0.5% only available as special preparation from selected pharmacies, recommended for infants less than 1 year old), ointment 1%. Duration of action: 1-2 weeks.

Although the advantage of phenylephrine is that it does not cause blurring of vision like the parasympatholytics it tends not to dilate the pupil well enough unless used in combination with parasympatholytics. Some doctors chose to use both types of drops mixed in the same bottle: for example, 0.8% tropicamide + 5% phenylephrine

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